Project Summary: We propose to develop software to accurately simulate the MRI process, from spin excitation through image reconstruction, with rigorous implementation of the physical laws of MRI and accurate 3D representations of human anatomies, field distributions, and tissue spin magnetization and relaxation properties. The output will be realistic images, accurately representing signal, contrast, and noise distributions, static and RF field-related artifacts (among many other things) for the chosen sample, sequence, and fields. This will facilitate an intermediate, low-cost stage of evaluation, revision, and optimization before implementation of novel hardware, pulses, or sequences on an MRI system. For use with the software, we will provide a library of realistic static, gradient, and RF magnetic field distributions at several field strengths from 1.0 to 9.4 tesla, human anatomies, basic pulse sequences, and reconstruction methods. The user will also be able to input specially-designed subject anatomies, different tissue spin magnetization/relaxation properties, novel field distributions at any field strength, exotic pulse sequences, and/or new reconstruction methods. The software will also calculate the head-average and maximum local Specific energy Absorption Rates (SAR) for the desired pulse sequence for comparison to safety regulations. All will be freely available and open-source so researchers can make modifications for their specific needs. Relevance: Advances in MRI have made it an invaluable tool in medicine and research in recent years, saving both lives and cost of care with accurate non-invasive diagnostic, anatomic, and physiologic information. Continual effort is devoted to improving MRI by using higher static field strengths, and thus higher RF field frequencies, but distortions of the applied fields by the human body present significant obstacles to the advancement of MRI. With the tools we will provide it will be possible to perform critical research, development, and safety evaluation of pulses, sequences, and hardware at any field strength using computers to reduce the need for magnet time and the cost of implementing untested prototypes on actual MRI systems. It will also make an excellent tool for MR education and training. This should greatly reduce the cost and increase the speed and accessibility of advancement of MRI and allow for valuable MRI system time to be dedicated more to important medical and scientific studies and less to evaluation of technical developments.